The major objective of this proposed research is to determine the reason why actin filaments contain tropomyosin always in muscle and often in non-muscle cells, even when corntraction is not regulated by troponin. Another study deals with some aspects of the muscle calcium switch that have not yet been clarified. The following studies are planned. Investigation of the activating effects of tropomyosin on smooth muscle and Limulus actomyosin systems to determine whether that effect is the result of potentiation caused by cooperative effects of myosin attached to tropomyosin actin filaments. Investigation, whether potentiation occurs in non-muscle actomyosin systems and what its physiological advantages are. Studies to determine the mechanism of potentiation, acceleration of myosin attachment to actin filaments, the role of myosin phosphorylation, the role of troponin in addition to tropomyosin, the structural basis for potentiation, the number of myosin molecules needed to achieve the cooperative effect. The questions relating to the calcium switch all deal with the complete switch i.e. on regulated actin filaments as distinct to measurements on troponin C or isolated troponin. We want to know whether 2 out of 4 calcium ions cooperate in activation, and to what extent several troponin molecules cooperate and whether, at low calcium, there exist intermediate states of activation.